Topband: Receiver problems.

[K3BU@aol.com]

KC1SX:

>Ah, but you missed something here, Yuri.  A decent high-end rig will have a
>sensitivity of about -140 dBm.  It would also have an IMD dynamic range of
>about 100 dB at a 20 kHz spacing.  That would be a 30 dB over S9 signal
>(assuming Collins standard for S-meters).
>
>In other words, a strong signal doesn't have to be within your roofing
>filter to be a problem.  In fact, the "ultimate" DR of a rig is usually only
>a few dB better than at 20 kHz spacing.  On the FT-1000MP, the IMD DR was
>100 dB at 100 kHz spacing, so multiple S9+30 dB signals could create IMD
>products in your receiver from anywhere in the band.

..... snipped

>Note that some of the above rigs would experience a _blocking_ response from
>an S9+30 signal that was more than 5 kHz away.
>
>You can expect that I will be examining this issue further.

    We know that radio that chokes with 9+30 (good "average" strong) signal few 
kHz away will also choke with signals that are spaced further away, depending 
on the design of radio's front end, mixers and IF circuits. The point is that 
contester (and other serious hams) are concerned with QRM and intermods from 
strong signals that are few hundreds of Hz away from receiving frequency (and 
anything beyond that), especially on low bands, where that situation is more 
pronounced. (Assuming roofing filters, etc, do their job.) Europeans have a 
RF hell on low bands from BC stations.

    Good receiver would be able to handle (amplify, mix) strong signals 
without (or with little) distortion ahead of filtering. Back in old days some 
people had RF stages made with higher power tubes that wouldn't get driven 
into nonlinear region and will not produce intermods ahead of filters. I 
don't want to get into discussion of proper receiver design, but the bottom 
line is, I'd like to know how close to my receiving frequency let's say S9+30 
signal can get before it starts bothering my S0 signal. That is much more 
descriptive of RX properties than 20 kHz separation.
 
    I was slowly gathering equipment to do tests like that and will check 
some of the radios and select the best ones for further modifications. Phase 
noise is another pain in the RX.

   The crude test of RX performance can be done by setting the TX into dummy 
load positioned to produce S9+30 signal on the RX with antenna on and on 
quiet band. Send bunch of dits and see how close you can get to TX frequency 
before you start hearing hash/crud on RX. Don't be surprised to hear hash 
with just PTT on, without keying. That is your friendly and noisy synthesizer 
:-)

    There are various ways to improve RX performance: Xtal filters at the 
antenna, Q-multipliers in the RF preselectors, phasing units, hi-Q passive 
circuits, high power RF and mixer devices, cascaded filters, etc.

   One goal of extreme contest receiver (setup) is to be able to come close 
to own CQing frequency (on the same band) while calling CQ and to be able to 
tune the band for new ones . With some antenna selection and phasing, 
combined with clean TX and crunch proof RX it is possible to do and has been 
done (WW2Y, CN8WW). That's my present challenge. Them Drake lines look better 
and better :-) - they are excellent vehicles for modifications.

    Clickses and splatters are another problem, but that is in the TX 
department and no receiver can fix that. Number of trashy signals in the 
contests seems to be growing (part of "keep'em away" from "my" frequency?). 
There was one signal on 10m in CQ WW about 70 kHz wide that I couldn't even 
find who or where it was. 

    ARRL reviews should reject radios with dirty keying excessive phase 
noise, there is no excuse to have that with today's designs!

Yuri, K3BU
Tesla RC N2EE


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